# 翻转一棵二叉树。

# 示例：

# 输入：

#      4
#    /   \
#   2     7
#  / \   / \
# 1   3 6   9
# 输出：

#      4
#    /   \
#   7     2
#  / \   / \
# 9   6 3   1


class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution:
    def invertTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
       if not root:
            return root
        root.left,root.right = self.invertTree(root.right),self.invertTree(root.left)
        return root
